The invention relates to a system for controlled lowering of an elongated body such as a tube or cable into a volume of water such as a lake or sea, from a relatively high level, especially the water level of said volume of water, to a relatively low level, especially the bottom under said volume of water, comprising an elongated tubular guide which is connected to one or more buoyancy bodies, and braking means for braking the movement of said elongated body through said tubular guide, said braking means incorporating at least one braking unit comprising:
a flexible tubular inflatable body, the outer wall being supported by said tubular guide and the inner wall of which acts as braking surface,
a valve unit for inflating or deflating the inflatable body whereby the valve unit is connected to a source of pressurized fluid and is controlled by a control unit.
Such a system is known from the US specification U.S. Pat. No. 5,575,590. In this document air is used to inflate or deflated the inflatable bodies of the braking units. Each braking unit is connected to an individual source of pressured air and the pressure inside the inflatable body of each of the braking units is preset independent of the other braking units on a value which is dependent on the depth at which the respective braking unit thereafter has to function. Because of the increasing hydrostatic pressure the air or gas in the inflatable body is compressed more and more with increasing depth. As a result the contact surface between the inflatable body and the tube becomes smaller and smaller so that in fact in each braking unit only a part of the initial braking surface will be active. Controlling the actual braking force under these circumstances in the prior art system is rather difficult or even impossible.
The object of the invention is now to eliminate the above indicated disadvantage.
In agreement with said object the invention now provides a system of the above indicated type which according to the invention is characterized in that the fluid inside each inflatable body has a specific weight equal to or at least nearly to the specific weight of the surrounding water.
It is especially preferred that the fluid inside each inflatable body consists of the same water as the surrounding water.
In one further development each valve unit is connected to a separate fluid reservoir attached to the corresponding braking unit and acting as source of pressurized fluid for said valve unit. Each braking unit is now able to operate independent of the others. The large number of reservoirs could be a disadvantage of this embodiment.
In another development a number of valve units (preferably all valve units) are connected to a fluid conduit which extends along the tubular guide and is connected to one fluid reservoir acting as source of pressurized fluid for said valve units. Only one reservoir is needed in this embodiment, however, a fluid conduit is necessary between each valve unit and the common reservoir.
The control communication between the control center and the valve units can be performed in various ways using data transfer along electrical connections, along acoustical communication paths, using radio waves, etc.
In all known prior ad cases pressurised air is used as fluid for inflating or deflating the inflatable bodies of the braking units and for determining in correspondence therewith the applied braking force. Assume a water depth of 2000 m and assume a desired control range of 50 cm water column. In that case 4000 braking units would be necessary each capable of functioning at a different depth and at a different hydrostatic bias pressure.
With increasing water depth the controllability of the valve units becomes more and more a problem. The valve unit which has to operate for instance at 1000 m depth should have a pressure control range between 999.75 m and 1000.25 m water column. Such a control range poses a serious problem. This problem will be twice as serious for the most lower valve unit which has to operate in the pressure range between 1999.75 m and 2000.25 m water column. An accurate control within such a small control range under such relatively high bias pressure of the surrounding sea water is hardly performable.
The whole problem is caused by the depth dependent hydrostatic pressure. The question how to eliminate this problem appeared to be surprisingly simple. The hydrostatic pressure acting on the outside of the inflatable bodies baa to be eliminated by the pressurizing control fluid inside the inflatable bodies. To obtain such a situation a preferred embodiment of the system is characterised in that the fluid has a specific weight equal to the specific weight of (sea)water. It is especially preferred that the fluid is (sea)water.
By using (sea)water as fluid the hydrostatic pressure difference is completely eliminated at all depths.
In a preferred system, in which the abovementioned fluid is (sea)water, the number of braking units is equal to one, said one braking unit comprising:
an elongated non stretchable outer tubular wall
an elongated flexible tubular inflatable body, the outer wall of which is supported by said outer tubular wall and the inner wall of which acts as braking surface,
a valve unit connected to a pump unit through which (sea)water can be pumped in or out the inflatable body to obtain the required braking stress.
The abovementioned elongated outer wall and the elongated flexible tubular inflatable body preferably extends from just below sea level to just above bottom level. In that case the whole tube is guided and controlled by only one combined component.
If for reasons which will not be discussed here it is preferred to use two or more braking units then each of the braking units comprises
a non stretchable outer tubular wall
a flexible tubular inflatable body, the outer wall of which is supported by said outer tubular wall and the inner wail of which acts as braking surface,
whereby all outer tubular walls are mechanically connected in series,
whereby all flexible tubular inflatable bodies and a valve unit are connected in series by suitable conduits, and
whereby furthermore the valve unit is connected to a pump unit through which (sea)water can be pumped in or out the series connected inflatable bodies to obtain the required braking stress.
The pump can be of a rather simple design which only has to be able to generate a pressure in the desired control pressure range, in the above example a pressure between 0 and 50 cm water column.